This protocol aims at testing our hypothesis that free radicals and active states of oxygen are of importance in the process of tumor promotion. This will be done by evaluating whether known mouse skin tumor promoters produce free radicals and/or active states of oxygen in vivo in mouse skin, and whether known inhibitors of mouse skin tumor promotion result in a decrement in the extent of formation of these reactive species. The approaches to identifying the presence or sequelae of reactions by free radicals or active states of oxygen in the skin of mice being treated with various tumor prompters are: 1. Indirect measurement of hydrogen peroxide in vivo by assay of catalase activity in skin obtained from animals injected with 3-amino-1,2,4,-triazole (ATZ). 2. Assay of cholesterol oxidation products by gas chromatography. 3. Assessment of the reduction of nitro-blue tetrazolium in phagocytes isolated from mouse skin as an index of in vivo activation. 4. Use of fluorescence technique to identify amino-iminopropene derivates of lipid peroxidation decomposition products in extracts of the skin of mice chronically treated with tumor promoters. In each of these approaches listed above, experiments will be performed with and without different tumor promoters and with and without various compounds known to inhibit promotion.